Many different processes in the fabrication of microelectronic circuits require precise metering of process gas flows. A variety of common and exotic gases are used extensively in this field. This is just one of several technologies that require such control of gas flows. To obtain economically acceptable product yields, process gas mass flowmeters and controllers (MFCs) must have high accuracy and precision, as well as reliability. Further, such requirements will increase because of intense competition and the rapid rate of technological advancement in the semiconductor industry.
Mass flowmeter calibration is an essential component of any precision flowmeter system. Currently there are two principal approaches to the measurement of mass flowrates through a flowmeter calibration test. In one, a vessel is charged with the gas to be used, and the weight of the vessel is measured (as a function of time) as the gas leaves the vessel and passes through the unit under test. Since the weight of the gas, even at the start of outflow of the gas, is significantly less than that of the weight of the vessel, the accuracy of weight measurement is difficult to achieve. In addition, the vessel must be disconnected from the unit under test in order to make the measurements since any connecting element affects the weight.
Another system, as utilized by the Bureau of Standards, utilizes careful measurement of temperatures, pressures and volumes of components during flow such that all of these variables are utilized to compute (via conventional gas equations) the gas flow. Accordingly, the system is very complex.
Accordingly, it is an object of the present invention to provide a device for the accurate determination of mass flow of a gas wherein the device remains connected to apparatus into which the gas is flowing.
Another object of the present invention is to provide a mass flow measuring device having a significantly greater accuracy than devices of the prior art.
A further object of the present invention is to provide a device for mass flow that relies only upon simple apparatus for measuring weight change of a vessel, where the weight change of gas in the vessel is a major part of the weight measured in the system.
It is also an object of the present invention to utilize a gravimetric measurement of weight change of a substantially buoyant body for the accurate measurement of gas mass flow.
Also, it is an object of the present invention to provide a device for determining mass flow in a significantly reduced time; for example, in a few minutes as contrasted to several hours or days.
It is still another object of the present invention to provide a device for determining mass flow of gases where the gases are extremely corrosive.
These and other objects of the present invention will become apparent upon a consideration of the drawings referred to below, and the detailed description thereof.